Various bearing assemblies and methods of lubricating these assemblies can be found in many applications. Typical bearing assemblies include a pair of bearing members that are movable relative to one another. More specifically, a bearing or bushing is typically a cylindrical shell, with or without a flange, installed in a structural housing in conjunction with a mating shaft, pin, or bolt. Bearings having pinned joints typically are used for cyclic and oscillatory motion, such as used in landing gear joints, control surface hinge points, and actuating linkages.
The advantages of bearings are numerous. Bearings are relatively economical replacement elements which extend the useful life of major structural elements by providing protection from wear, corrosion, deformation, and other service-limiting or failure-initiating damage. The two basic considerations for successful service performance of a bearing are strength and life. Strength is the capability of the bearing to resist deformation and structural failure under static load. The life of the bearing is determined by its ability to resist wear, fretting, galling, and seizure under cyclic or oscillating motion.
FIG. 1A shows a typical bearing structure assembly 100, wherein grease 106, such as Royal MS 11 manufactured by Royal Lubricants, Inc. of East Hanover, N.J., is applied between bearing assembly members comprising a bearing 101 and pin 103 to lubricate the members as they move relative to one another. Specifically, the grease 106 is disposed between the inner surface 102 of the bearing 101 and the outer surface 104 of the pin 103. While grease is suitable for many applications, extreme pressure and movement by the bearing assembly members may cause the grease to lose its lubricity, which may lead to overheating and eventual failure of the bearing assembly and surrounding structures.
One application where this may occur is on a landing gear apparatus of an aircraft, and more particularly on a truck pivot joint bearing assemblies for an aircraft landing gear. The bearing assemblies used in this type of application typically include a relatively thick coating of grease between the moving surfaces of the bearing. This is acceptable for most circumstances because the dynamic bearing pressures and sliding velocities of the moving surfaces are relatively low. However, rough landing conditions, such as runways with deep cracks, crevices, potholes, and/or uneven surfaces, can cause excessive use of and wear on the truck pivot joint bearing assemblies from the rapid shocks and extreme oscillations these surface abnormalities transfer to the bearing assemblies. In some cases, it has been discovered that these rough conditions create as much as ten (10) times more energy than is generated under normal conditions. This additional energy is absorbed as heat by the bearing assemblies, which accounts for the damage that occurs when using conventional bearing assemblies in these environments. As a result, the high temperatures generated in these conditions cause the grease packed between the moving surfaces of the bearing assemblies to melt away and thus lose its effective lubricity. This is shown in FIG. 1B, wherein the grease 106 is no longer occupying the entire space between the opposing bearing members 101, 103, which allows the inner surface 102 of the bearing 101 to have intimate contact with the outer surface 104 of the pin 103.
As the grease loses lubricity and the surfaces 102, 104 of the bearing assembly 100 begin to directly move against each other, severe damage or failure of the bearing assembly components is likely. To prevent failure of the bearing assembly, time-consuming maintenance must be performed more frequently, which further adds to the cost of maintaining the aircraft in addition to the lost profits while the aircraft is under maintenance or repairs. Thus, it is desirable to provide a lubricant to bearing components that is more resistant to dynamic bearing pressures and heat generation in extreme conditions.
FIG. 2A shows another type of conventional bearing assembly. In particular, the bearing assembly 110 includes a bearing 111 and a pin 113 having opposing surfaces 112 and 114, respectively. A dry, greaseless coating material 116 is applied to a desired surface by spraying or coating the surface, such as the inner surface 114 of the bearing 111. The greaseless, self-lubricating material 116 occupies the space between the opposing bearing surfaces 112, 114 such that the surfaces are only separated by the greaseless material. One example of a greaseless self-lubricating material 116 is a polyester, thermosetting, resin-based material incorporating polytetrafluoroethylene or TEFLON® particles, such as the material manufactured under the name KARON by Kamatics Corporation of Bloomfield, Connecticut. This type of material is proclaimed as self-lubricating, meaning no external grease is required to lubricate the bearing assembly 110. Indeed, greaseless lubricants such as shown in FIG. 2 were designed to overcome the disadvantages of grease lubricants shown in FIG. 1, particularly in terms of load capacity and service life. Other types of greaseless lubricants are described in U.S. Pat. Nos. 3,929,396 and 3,996,143.
In particular, greaseless, self-lubricating materials operate at lower friction levels, which reduces the heat generated during operation. If heat does build up, the polytetrafluoroethylene particles typically expand more rapidly than the underlying surfaces to fill the space between the bearing surfaces, so that frictional contact between bearing surfaces 112, 114 is thwarted or delayed, at least temporarily. Conventional practice teaches that a thicker coating of the greaseless material 116 will provide more lubrication for the bearing 110. While true for most applications, the extreme loading conditions mentioned above may cause the greaseless material to break down. And because the greaseless material 116 allows the bearing surfaces 112, 114 to be essentially in contact with each other separated only by the greaseless lubricant, any reduction in the thickness of the greaseless lubricant can reopen the space between the bearing surfaces, which can damage the bearing or cause the bearing to fail. This is shown in FIG. 2B, wherein the greaseless material 116 has worn down in certain areas, which allows unwanted and damaging rattling or movement between the bearing assembly members 111, 113. Thus, while greaseless lubricants provide advantageous qualities over grease lubricants, there is a need to provide a bearing assembly having a lubricant that offers even better wear and heat resistance, which leads to longer operational life of the bearing. There is also a need to provide a bearing assembly having a lubricant that is resistant to extreme loading conditions, such that the lubricant provides longer protection to the bearing in these environments compared to protection from conventional lubricants before service is required.